As a technology of extending the band of a digital audio signal, such a technology is known that a predetermined nonlinear process is performed on the digital audio signal to be inputted, to thereby generate a higher-frequency signal component than the digital audio signal to be inputted (refer to a patent document 1 and a non patent document 1). For example, in the technology disclosed in the patent document 1, the higher-frequency signal component than the digital audio signal to be inputted is generated by performing full-wave rectification, which is to take an absolute value of the digital audio signal to be inputted.
Patent document 1: Japanese Patent Application Publication NO. 2003-317395
Non Patent document 1: Ronald M. Aarts and Erik Larsen and Daniel Schobben, “IMPROVING PERCEIVED BASS AND RECONSTRUCTION OF HIGH FREQUENCIES FOR BAND LIMITED SIGNALS”, Proc. 1st IEEE Benelux Workshop on Model based Processing and Coding of Audio (MPCA-2002), Belgium, Nov. 15, 2002, pp 59-71